Qt applications fail due to xcb plugin mismatch

[lheckemann]

Issue description

% nix-shell -p openshot-qt

[nix-shell:~]$ openshot-qt
Loaded modules from current directory: /nix/store/0r6czdfcgfwqgar2sbz5zhrbysrmkn80-openshot-qt-2.1.0/lib/python3.5/site-packages
      launch:INFO ------------------------------------------------
      launch:INFO    OpenShot (version 2.1.0)
      launch:INFO ------------------------------------------------
This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

Available platform plugins are: minimal, offscreen, xcb.

Reinstalling the application may fix this problem.
Aborted

[nix-shell:~]$

Technical details

• System: 17.03beta320.ade5837 (Gorilla)
• Nix version: nix-env (Nix) 1.11.7
• Nixpkgs version: "17.03beta617.f3a9ccc"

[lheckemann]

Works fine when installed using nix-env -iA nixos.openshot-qt, but both should be possible, right?

[AndersonTorres]

Incidentally there is a new version of openshot: #22710

(edited a typo, many thanks @lheckemann !)

[lheckemann]

Wrong issue number? I guess you meant #22710

[edolstra]

I have the same problem with krita (installed via nix-env):

$ krita
This application failed to start because it could not find or load the Qt platform plugin "xcb"
in "".

Available platform plugins are: minimal, offscreen, xcb.

Reinstalling the application may fix this problem.
Aborted

Installing it via environment.systemPackages does work.

strace shows the following:

open("/run/current-system/sw/lib/qt5/plugins/platforms", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
open("/nix/store/32s4bd5mficwpp6064s5rvkpvhzycbkn-qtbase-5.6.2/lib/qt5/plugins/platforms/libqxcb.so", O_RDONLY|O_CLOEXEC) = 3
open("/nix/store/96xragmk6ap2y988qpw50ck7qwfg511j-qtbase-5.7.1/lib/qt5/plugins/platforms/libqxcb.so", O_RDONLY|O_CLOEXEC) = 3

So it's opening two mismatching versions of libqxcb.so, where one is probably discovered via /run/current-system/sw/lib/qt5/plugins/platforms.

[abbradar]

Related discussion: #24133

For now one should remove everything that installs Qt plugins (not input ones, they are good) from their environment to fix this -- usually that's qtbase, qtEnv or other Qt packages.

cc @ttuegel

[ttuegel]

I am working on making Qt packages entirely insensitive to their environment.

[edolstra]

@ttuegel What would be the implications of that? Isn't the plugin mechanism necessary for certain things (like picking up the open dialog box of the desktop environment, theming etc.)?

[ttuegel]

Isn't the plugin mechanism necessary for certain things (like picking up the open dialog box of the desktop environment, theming etc.)?

Right now, I have patched our Qt libraries to go spelunking through NIX_PROFILES in addition to QT_PLUGIN_PATH. I'm not going to remove plugin support, I'm just going to stop going through NIX_PROFILES looking for every possible plugin. 😃 All the desktop integration and theming plugins are currently provided by Qt or KDE anyway, but QT_PLUGIN_PATH will still work should it be necessary.

[glaebhoerl]

FWIW if anyone wants a quick and ugly hack to just get an application (in my case kolourpaint) to hopefully start without having to remove or reinstall a bunch of packages:

$ ls /nix/store/*qtbase*/lib/qt5/plugins/platforms/libqxcb.so
/nix/store/0igx2hwicb18di1mx65qh3jrpypvc40a-qtbase-5.6.1-1/lib/qt5/plugins/platforms/libqxcb.so 
/nix/store/9kmi6a4pba9g45wr4c1ny5kasc70jbx9-qtbase-5.6.2/lib/qt5/plugins/platforms/libqxcb.so
/nix/store/3qz77v287y42s3a9l1745551idfcdflr-qtbase-5.8.0/lib/qt5/plugins/platforms/libqxcb.so    
/nix/store/f2680a9j60qi724k7dgphxamlqsq933x-qtbase-5.6.1-1/lib/qt5/plugins/platforms/libqxcb.so

(choose the one that seems appropriate, or just trial and error)

$ NIX_PROFILES=/nix/store/3qz77v287y42s3a9l1745551idfcdflr-qtbase-5.8.0 kolourpaint

(May or may not fail for applications with more complicated dependencies than kolourpaint, did not try.)

[glaebhoerl]

...hm. I'm not 100% sure if this issue is the culprit, but I rebooted and KDE itself failed to start (error message something like "core shell components could not be found, this is a packaging problem, contact your distro"). I did nix-env -e kolourpaint and rebooted again and then it worked.

[ttuegel]

This is fixed in master: Qt applications know about the QPA plugins they are built with.

[CMCDragonkai]

@ttuegel I think I'm getting this error at #27790 is there quick fix for people still on 17.03? @glaebhoerl 's trick did not work for me.

[edolstra]

@ttuegel I still this error when running 17.09-based Qt packages on NixOS 17.03 (e.g. Krita or Tribler). For Tribler, I added this line to the wrapper:

        --set QT_QPA_PLATFORM_PLUGIN_PATH ${qt5.qtbase.bin}/lib/qt-*/plugins/platforms \

which fixes it. Is that the recommended solution?

[CMCDragonkai]

@edolstra Would that flag be relevant to other QT apps as well like #27790

[ttuegel]

For Tribler, I added this line to the wrapper which fixes it. Is that the recommended solution?

I believe setting that variable in the wrapper will break the application under KDE.

It should not be necessary for you to do anything. If it is, then there is a bug I need to fix in our modified Qt plugin loader. Just to check, you have the package installed in environment.systemPackages or by nix-env, right?

[edolstra]

@ttuegel I was running it as ./result/bin/tribler (so not installed at all).

[ttuegel]

I was running it as ./result/bin/tribler (so not installed at all).

Unfortunately, that is not expected to work. I don't know how to make programs work from the store while also integrating correctly into desktop environments like KDE or LXDE. (This is a really terrible trade-off and I will entertain any suggestion to fix it.) A work-around is to set QT_QPA_PLATFORM_PLUGIN_PATH in shellHook so that at least nix-shell will work.

[jirkadanek]

What helped me was either to 1) declare the application in /etc/nixos/configuration.nix 2) run nix-env -u --leq, the doc says for this parameter

In addition to upgrading to newer versions, also “upgrade” to derivations that have the same version. Version are not a unique identification of a derivation, so there may be many derivations that have the same version. This flag may be useful to force “synchronisation” between the installed and available derivations. -- https://nixos.org/releases/nix/nix-1.7/manual/#sec-nix-env

[jluttine]

FWIW, I'll add a comment how I fixed this in my case. I ran the failing command with LD_DEBUG=libs some_failing_command and then read from the last few lines what library it failed to find. Then added that to the environment and repeat until the command succeeds. For instance, in my case, adding xlibs.libX11 and xlibs.libXi to the environment solved the issue.

Background: I have FHS user env for Conda and when I launched IPython (ipython --pylabs) installed by Conda, I got the error: This application failed to start because it could not find or load the Qt platform plugin "xcb" in "". So I just needed to add all relevant libraries to this FHS user env. This is probably very different scenario than the OP (or others) had, but just thought I'd share this here in case someone has a similar issue as I did and looks for help here.

[aakropotkin]

Hey I was just struggling with this for a few hours trying to run VirtualBox.
This has some obvious flaws that will not make this a great solution for everybody; but if you're in a bind you can give this a shot.

My fix was :

# In `configuration.nix`
{ config, lib, pkgs, options, ... }:
{
  virtualisation.virtualbox.host.enable = true;
  environment.pathsToLink = ["${pkgs.xorg.libxcb}/lib/"];
}

[timbertson]

My previous workaround of exporting QT_QPA_PLATFORM_PLUGIN_PATH in my shell caused my (fedora system installed) vlc to segfault on startup. So I'd definitely vote for qt apps to be self-contained rather than integrating with the desktop, if this is the outcome.

[jmercouris]

I just upgraded to 20.09 and am running into this issue with my KDE applications. How to fix?

[fadenb]

Same here, experiencing this since upgrading to 20.09.
In my case, electrum is installed system wide.

→ electrum                                                                                                                                                                        ~
qt.qpa.plugin: Could not load the Qt platform plugin "xcb" in "" even though it was found.
This application failed to start because no Qt platform plugin could be initialized. Reinstalling the application may fix this problem.

Available platform plugins are: wayland-egl, wayland, wayland-xcomposite-egl, wayland-xcomposite-glx, eglfs, linuxfb, minimal, minimalegl, offscreen, vnc, xcb.

[1]    6077 abort (core dumped)  electrum

[bhipple]

Perhaps try upgrading everything you have imperatively installed via nix-env? You may have an old ver in your ~/.nix-profile and a new ver on the system.

nix-env --upgrade

[jmercouris]

I have installed nothing imperatively.

[Luz]

I have not found my desired solution in this thread, but in that other one. So I'd like to link it to here, in case anyone else is stumbling upon this.
I suggest to add the following line to the nix-shell file (in my case to a default.nix):
QT_QPA_PLATFORM_PLUGIN_PATH="${qt5.qtbase.bin}/lib/qt-${qt5.qtbase.version}/plugins";